Kellyanne Conway during a meeting in the Roosevelt Room of the White House on Tuesday.Credit
Stephen Crowley/The New York Times

WASHINGTON — The White House adviser Kellyanne Conway’s public endorsement of Ivanka Trump’s product line appears to be “a clear violation of the prohibition against misuse of position,” and she should be disciplined, the federal government’s chief ethics watchdog wrote this week in a letter to the White House.

Although President Trump’s press secretary, Sean Spicer, said last week that Ms. Conway had been “counseled” about her statement, the White House has not clarified what that means. Experts in government ethics say that typically, offenses like Ms. Conway’s would result in a letter of reprimand, though it could be grounds for termination. It is up to the head of each agency — in this case, the president — to determine the punishment.

The Office of Government Ethics “has not yet received notification of any disciplinary or other corrective action against Ms. Conway,” Walter M. Shaub Jr., director of the office, wrote in a letter to Stefan C. Passantino, the deputy counsel to the president who is the designated ethics officer at the White House.

“There is strong reason to believe that Ms. Conway has violated the standards of conduct and that disciplinary action is warranted,” the letter said.

The ethics office’s letter adds to the troubles piling up on Ms. Conway. On Monday, she was dispatched to the television cameras to say that Michael T. Flynn, the national security adviser, had the full confidence of Mr. Trump, just hours before Mr. Flynn resigned. On Tuesday, she appeared to respond to criticism of her public performances, posting on Twitter that she serves at the pleasure of the president. “His message is my message. His goals are my goals. Uninformed chatter doesn’t matter,” she wrote.

I serve at the pleasure of @POTUS. His message is my message. His goals are my goals. Uninformed chatter doesn't matter.

Last week, Representative Jason Chaffetz of Utah, chairman of the Oversight and Government Reform Committee, called Ms. Conway’s endorsement of Ms. Trump’s product line “wrong, wrong, wrong,” and he called on the ethics office to investigate. His response was all the more striking given that Mr. Chaffetz, a Republican, had brushed off previous calls to investigate Mr. Trump or those around him, despite his committee’s broad investigative powers.

Ms. Trump, the president’s daughter, markets fashions under her name, and opponents of Mr. Trump have called for a boycott of those and other products and services bearing the family name. Nordstrom, the department store chain, said recently that it would no longer carry her products, one of several retailers that have curtailed or ended their business with her line.

On Thursday, Ms. Conway, whose title is counselor to the president, gave an interview to Fox News from the White House briefing room, with the White House insignia visible behind her, defending the president and his daughter.

“Go buy Ivanka’s stuff is what I would say,” she said. “I’m going to give a free commercial here: Go buy it today, everybody; you can find it online.”

Federal ethics rules on conflicts of interest specifically exempt the president and vice president. But for any other executive branch employee, the rules prohibit using public office for personal gain, or “for the endorsement of any product, service or enterprise, or for the private gain of friends, relatives or persons with whom the employee is affiliated in a nongovernmental capacity.”

Mr. Shaub, who was appointed by President Obama in 2013 to a five-year term to head the ethics office, has said that Mr. Trump has not gone nearly far enough in separating himself from his business interests.

Correction: February 14, 2017

An earlier version of this article misspelled the surname of the director of the Office of Government Ethics. He is Walter M. Shaub Jr., not Schaub.

A version of this article appears in print on February 15, 2017, on Page A11 of the New York edition with the headline: Ethics Office Says Adviser Broke the Rules. Order Reprints|Today's Paper|Subscribe